Synthesis and antiviral activity of novel D- and L-2′-azido-2′,3′-dideoxyribofuranosyl-4′- thiopyrimidines and purines

Article abstract of DOI:10.1016/S0960-894X(01)00006-3

Novel D- and L-2′-azido-2′,3′-dideoxyribofuranosyl-4′- thiopyrimidines and purines have been synthesized starting from L-xylose and D-xylose, respectively. Among synthesized compounds tested against several viruses such as HIV-1, HSV-1, HSV-2, and HCMV, D-β-N⁶-methyladenine (ent-22a) and D-α-N⁶-methyladenine (ent-22b) analogues were found to exhibit significant anti-HCMV activity.

Full text of DOI:10.1016/S0960-894X(01)00006-3

Bioorganic & Medicinal Chemistry Letters 11 (2001) 599±603
Synthesis and Antiviral Activity of Novel D- and
L-20-Azido-20,30-dideoxyribofuranosyl-40-thiopyrimidines
and Purines
Hea Ok Kim,a Yun Ha Kim,b Hwal Suha and Lak Shin Jeongb,*
aCollege of Medicine, Yonsei University, Seoul 120-752, South Korea
bLaboratory of Medicinal Chemistry, College of Pharmacy, Ewha Women's University, Seoul 120-750, South Korea
Received 24 November 2000;accepted 6 January 2001
AbstractÐNovel d- and l-20-azido-20,30-dideoxyribofuranosyl-40-thiopyrimidines and purines have been synthesized starting from
l-xylose and d-xylose, respectively. Among synthesized compounds tested against several viruses such as HIV-1, HSV-1, HSV-2,
and HCMV, d-b-N6-methyladenine (ent-22a) and d-a-N6-methyladenine (ent-22b) analogues were found to exhibit signi®cant anti-
HCMV activity. # 2001 Elsevier Science Ltd. All rights reserved.
Introduction
30-azido-30-deoxy-40-thiothymidine (1) with no anti-HIV
activity and Marquez and Jeong7,8 had reported the
synthesis and conformations of 20,30-dideoxy-20- or 30-
¯uoro-40-thiouridines and cytidines (2a and 2b) in which
20-`down'-¯uoro cytidine analogue showed weak anti-
HIV activity in CEM cells (Fig. 1). From the X-ray
crystallographic analyses, conformations of 2a and 2b
were found to be the same as those of the corresponding
40-oxonucleosides.7,8 Therefore, since the synthesis of
20,30-dideoxy-40-thionucleosides in which the 20-position
was substituted with azide has never been reported so
far, it is interesting to synthesize this class of nucleosides
and to evaluate them for antiviral activity. It is also inter-
esting to synthesize d- and l-nucleosides and to com-
pare their antiviral activities. Here, we report the ®rst
synthesis and antiviral activity of d- and l-20-`down'-
azido-20,30-dideoxy-40-thionucleosides (3 and ent-3) uti-
lizing our versatile intermediate, 40-thioarabitol deriva-
tive9 developed by our laboratory.
Since 30-azido-30-deoxythymidine (AZT, zidovudine)1
has been identi®ed as a potent anti-HIV agent, a num-
ber of 20,30-dideoxynucleosides having a substituent like
azide or ¯uorine on their 20 or 30 position have been
synthesized and many of them exhibited potent antiviral
activities.2 Based on these ®ndings, bioisosteric 40-thio-
nucleosides of the above-mentioned nucleosides have
been paid great attention by medicinal chemists for
the development of new antiviral agents, but their
synthetic diculties hindered them from being studied
for SARs.
With the development of ecient syntheses of 4-thiosugars
in the 1990s, several groups published improved syntheses
of d-20-substituted-40-thionucleosides and their biological
activities such as antiviral and antitumor activities,3À5 but
these nucleosides were studied as 20-deoxy-40-thio
analogues modi®ed only at the 20-position.
Since 20,30-dideoxy-40-thionucleosides with various sub-
stituents on their 20 or 30 position have rarely been
reported, we have been interested in the synthesis of
these classes of 40-thionucleosides as a part of our
ongoing e€ort for the development of new antiviral
agents. Several groups6 had reported the synthesis of
Results and Discussion
Our synthetic strategy was ®rst to synthesize the glycosyl
donor and then to condense with nucleosidic bases.
Synthesis of the glycosyl donor 9 is shown in Scheme 1.
d-Xylose was converted to the 4-thioarabitol derivative
4 according to the ecient synthetic procedure devel-
oped by our laboratory.9 Benzyl protecting group in 4
was removed by treating it with boron trichloride at
*Corresponding author. Tel.: +82-2-3277-3466;fax: +82-2-3277-2851;
e-mail: lakjeong@mm.ewha.ac.kr
0960-894X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(01)00006-3

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H. O. Kim et al. / Bioorg. Med. Chem. Lett. 11 (2001) 599±603
À78 ꢀC in 84% yield to give the diol which was selec-
tively silylated with TBDPSCl to a€ord monosilylate 5. To
remove the hydroxyl group at the C3 position of 5, com-
pound 5 was treated with phenyl chlorothionoformate to
give the thiocarbonate, which, without puri®cation, was
reacted with n-tributyltin hydride and triethyl borane to
give the deoxygenated product 6 in 70% yield. Com-
pound 6 was treated with methanolic ammonia to give
7. To introduce the azido group at the C2 position of 7,
Mitsunobu conditions (DPPA, DEAD, PPh3) were
employed, but were unsuccessful. Therefore, mesylation
(99%) of 7 followed by the treatment of the resulting
mesylate with sodium azide in DMF at 40 ꢀC, a€orded the
desired azide 8 (84%). As reported by our laboratory10 and
Yoshimura and his co-workers,5b azidation was proceeded
with pure SN2 type without participation of ring sulfur
unlike DAST ¯uorination. Oxidation of 8 with mCPBA at
À78 ꢀC followed by re¯uxing with acetic anhydride gave
the l-glycosyl donor 9.
For the synthesis of the l-cytosine analogues (11a and
11b) illustrated in Scheme 2, l-acetate 9 was condensed
with silylated N4-benzoylcytosine in the presence of
TMSOTf in 1,2-dichloroethane to give l-b-anomer 10a
(30%) and l-a-anomer 10b (31%) after separation by silica
gel column chromatography. Anomeric con®gurations of
1
10a and 10b were assigned by H NOE experiment and
the comparison of chemical shifts and coupling constants
0
1
of their H NMRs. Irradiation of 2 -H of compound 10a
gave small NOE e€ect (7.1%) on its 10-H, indicating b-
anomer, while larger NOE e€ect (9.1%) was observed in
the same experiment in the case of compound 10b, resulting
Figure 1. Rationale for the design of the target compounds.
1
in a-anomer. Besides NOE e€ect, similar H NMR pat-
terns of 10a 0and 10b were observed with those of 20-
substituted-2 ,30-dideoxy-40-oxonucleosides.11 For exam-
ple, coupling constant (Jtrans=1.2 Hz) of 10-H in 10a was
smaller than that (Jcis=4.9 Hz) of 10-H in 10b and
Scheme 1. Reagents and conditions: (a) BCl3, CH2Cl2, À78 ꢀC, 4 h,
84%;(b) RCl, imidazole, DMF, rt, 1.5 h, 76%;(c) i. PhOC(S)Cl,
DMAP, CH3CN, rt, 24 h. ii. Et3B, n-Bu3SnH, C6H6, 0 ꢀC, 70%;(d)
NH3, MeOH, rt, 24 h, 78%;(e) MsCl, Et 3N, DMAP, CH2Cl2, 40 ꢀC,
24 h, 99%;(f) NaN 3, DMF, 40 ꢀC, 72 h, 84%;(g) mCPBA, CH2Cl2,
À78 ꢀC, 3 h, 98%;(h) Ac 2O, 100 ꢀC, 24 h, 84%.
Scheme 2. Reagents and conditions: (a) silylated N4-benzoylcytosine,
TMSOTf, ClCH2Cl2Cl, rt, 24 h;(b) NH 3, MeOH, rt, 24 h;(c) n-
Bu4NF, THF, rt, 1 h.

H. O. Kim et al. / Bioorg. Med. Chem. Lett. 11 (2001) 599±603
601
chemical shift of the b-anomeric proton was up®eld rela-
tive to the a-anomeric proton. Furthermore, the 40-pro-
ton of a-anomer 10b appeared down®eld from that
observed for b-anomer 10a because of the deshielding
e€ect by cytosine and the 50-protons of 10a appeared
down®eld from those observed for 10b due to the same
deshielding e€ect. This typical 1H NMR pattern was
maintained in the case of other pyrimidine and purine
nucleoside derivatives which are shown in Scheme 3 and
4. Compounds 10a and 10b were treated with metha-
nolic ammonia followed by treating with n-tetra-
butylammonium ¯uoride to a€ord the ®nal l-cytosine
analogues 11a and 11b, respectively.12
Synthesis of the l-uracil derivatives (18a and 18b) and
l-thymine derivatives (19a and 19b) is shown in Scheme
3. l-Acetate 9 was condensed with silylated uracil and
thymine using TMSOTf as a Lewis catalyst to give the
inseparable a/b mixtures 12 and 13, respectively. Desi-
lylation of 12 and 13 with n-tetrabutylammonium ¯uo-
ride also resulted in inseparable a/b mixtures 14 and 15,
respectively. In order to separate the inseparable
anomeric mixture by silica gel column chromatography,
a/b mixture 14 was perbenzoylated at 70 ꢀC to a€ord
separable l-b-anomer 16a and l-a-anomer 16b, which
were deblocked separately using methanolic ammonia
to give the ®nal l-b-anomer 18a and l-a-anomer 18b,
respectively. According to the similar procedure used in
the preparation of l-uracil analogues 18a and 18b, l-
thymine analogues 19a and 19b were synthesized from
15 via benzoylates 17a and 17b, respectively.
Synthesis of the l-purine derivatives was accomplished
from the condensation of acetate 9 with silylated 6-
chloropurine at 60 ꢀC for 15 h to give inseparable a/b
mixture 20 as illustrated in Scheme 4. As reported pre-
Scheme 4. Reagents and conditions: (a) silylated 6-chloropurine,
TMSOTf, ClCH2Cl2Cl, rt to 60 ꢀC, 24 h;(b) n-Bu4NF, THF, rt, 1 h;
(c) MeNH2, MeOH, 80 ꢀC, 15 h;(d) NH 3, MeOH, 80 ꢀC, 15 h;(e) 1 M
NaOMe, HSCH2CH2OH, MeOH, re¯ux, 24 h.
Scheme 3. Reagents and conditions: (a) silylated uracil or thymine,
TMSOTf, ClCH2Cl2Cl, rt, 24 h;(b) n-Bu4NF, THF, rt, 1 h;(c) BzCl,
pyridine, 70 ꢀC, 24 h;(d) NH , MeOH, rt, 24 h.
3

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H. O. Kim et al. / Bioorg. Med. Chem. Lett. 11 (2001) 599±603
The d-pyrimidine and purine nucleosides (ent-11a, ent-
11b, ent-21a±23a and ent-21b±23b) were synthesized
according to the same procedure used in the preparation
of the corresponding l-pyrimidine and purine nucleosides
(Scheme 5). Antiviral assays for the synthesized d- and
l-nucleosides were performed against HIV-1 (MT-4
cells), HSV-1 (CCL81 cells), HSV-2 (CCL81 cells), and
HCMV (HEL299 cells). All tested compounds did not
exhibit signi®cant antiviral activities except against
HCMV. Among compounds tested, d-a-N6-methyladenine
analogue (ent-22b) was found to be the most potent in
AD-169 (EC50=19.15 mg/mL) and Davis (EC50=
25.79 mg/mL) infected cell lines without cytotoxicity up
to 100 mg/mL when compared to positive control, ganci-
clovir (EC50=0.98 mg/mL, in AD-169;EC 50=4.07mg/mL
in Davis) or phosphonoformate (EC50=9.79 mg/mL, in
AD-169;EC 50=86.54 mg/mL in Davis). It is not sur-
prising that d-a-isomer ent-22b showed anti-HCMV
activity in view that d-a-xylofuranosyl-40-thiopurine
nucleosides also exhibited potent anti-HCMV activity
as reported by Secrist and co-workers.6 d-b-N6-Methyl-
adenine analogue (ent-22a) was also found to show anti-
HCMV activity in AD-169 (EC50=26.09 mg/mL) and
Davis (EC50=90.21 mg/mL) infected cell lines without
cytotoxicity up to 100 mg/mL. l-b-6-Chloropurine (21a), d-
b-6-chloropurine (ent-21a), and d-a-6-chloropurine (ent-
21a) were found to exhibit toxicity-dependent anti-HCMV
activity. Other d- and l-nucleosides showed neither anti-
HCMV activity nor cytotoxicity.
In summary, we have accomplished the ®rst synthesis of
d- and l-20-`down'-azido-20,30-dideoxy-40-thiopyrimidine
and purine nucleosides. Although the tested compounds
did not exhibit excellent antiviral activity, it is believed
that observation of signi®cant anti-HCMV activity
allows this class of compounds to be the new template
for the development of new antiviral agents.
Acknowledgements
This work was supported by the grant from Department of
Health and Welfare, R&D Program (HMP-98-D-4-0057),
Korea. Authors also thank Dr. C.-K. Lee (KRICT) for
the antiviral testing.
Scheme 5.
viously,13 it was found that initially formed N3-isomer
cleanly migrated to N9-isomer on heating. After inse-
parable a/b mixture 20 was desilylated with n-tetra-
butylammonium ¯uoride, the mixture could be separated
by silica gel column chromatography to give l-b-anomer
21a (55%) and l-a-anomer 21b (34%). l-b-6-Chloro-
purine derivative 21a was converted to the l-b-N6-
methyladenine derivative 22a (65%) and l-b-adenine
derivative 23a (51%) by treating with methylamine and
ammonia in methanol at 80 ꢀC for 15 h, respectively.
Compound 21a was also converted to the l-b-hypox-
anthine derivative 24a (45%) by re¯uxing with mercapto-
ethanol and sodium methoxide. l-a-6-Chloropurine
derivative 21b was also converted to the l-a-N6-methyl-
adenine derivative 22b (63%), l-a-adenine derivative
23b (60%) and l-a-hypoxanthine derivative 24b (50%)
according to the same procedure used in the synthesis of
l-b-purine nucleoside derivatives.
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